The present invention relates to a projection exposure system used for accomplishing the lithographic processes for manufacturing, for example, semiconductor integrated circuits, liquid crystal display devices, etc., and particularly to such a projection exposure system with a mechanism for efficiently detecting and correcting magnification and other factors of a projection optical system used in the projection exposure system. The present invention is also applicable to a step-and-scan type of scanning projection exposure system in which patterns of a mask are projected for exposure onto one shot-field after another on a photosensitive substrate after each shot-field is moved to a scanning starting position in a stepping fashion.